The invention relates to a boron-free glass, preferably a neutral glass, which can be melted without the addition of boron-containing raw materials.
The term “neutral glass” is understood to mean glasses with very good hydrolytic resistance and very good acid resistance. Since these glasses thus have a “neutral” action, in that they scarcely impart glass constituents to the solutions, they can be used inter alia as primary packaging material in the pharmaceutical industry, in particular for injection solutions.
Table 1 summarizes the classifications of glasses with respect to the chemical resistance to water, acids and alkalis in accordance with the various standards.
TABLE 1Resistance to . . .WaterAcidAlkali(H)(S)(L)StandardDIN ISO 719DIN 12116DIN ISO 695SolutionDist. H2O6n HCl (semi-0.5m Na2CO3 +conc.)0.5m NaOHTemperature98° C.108° C.102° C.Duration1 hr6 hrs3 hrsUnitμg Na2O/g glassmg/dm2mg/dm2Class 1up to 31up to 0.7up to 75Class 2up to 62up to 1.5up to 175Class 3up to 264up to 15more than 175Class 4up to 620more than 15Class 5more than 620
The known commercially available neutral glasses, e.g. SCHOTT FIOLAX® 8412 and 8414 or SCHOTT DURAN® 8330 from Schott AG, Mainz, are classified in the group of borosilicate glasses, since they contain, more than 8% B2O3. These are glasses in hydrolytic class 1 and acid class 1 and in alkali class 2, referred to here for short as “1-1-2 glass”.
Although the boron oxide is present in SCHOTT FIOLAX® 8412 only in an amount of about 11%, the raw material disodium tetraborate pentahydrate amounts to approximately half the total costs for raw materials. The situation regarding raw materials for borosilicate glasses without sodium oxide, e.g. alkali metal-free glasses for LCD displays, is even more unfavorable, because in this case it is necessary to use the much more expensive raw material boron oxide (boric acid), which first has to be obtained by technical means from borax. The costs for the B2O3 glass component from the raw material boron oxide are seven times higher than the costs for B2O3 from the raw material disodium tetraborate pentahydrate.
The EU (European Union) has recently classified boric acid, diboron trioxide, disodium tetraborate anhydrite, disodium tetraborate decahydrate and disodium tetraborate pentahydrate as reprotoxic. As a result, it is necessary to comply with certain boundary conditions and to take certain precautionary measures during production using such raw materials.
Owing to the relatively high costs of boron-containing raw materials, the foreseeable shortage of suitable qualities and the current discussion relating to the reclassification of the toxicity of boron compounds, boron-free glasses are of interest as an alternative to the borosilicate glasses.
In addition to the very good chemical resistance, however, even further demands are imposed on neutral glasses.
By way of example, it has to be possible for the glass to be produced in conventional melting units, i.e. the viscosity of the melt cannot be excessively high—the working point (temperature at which the viscosity is 104 dPas, also referred to as VA or T4) should by no means exceed the maximum value of 1320° C. For energy-saving production, T4 should be as low as possible.
Although the thermal expansion in the range of 20° C. to 300° C. is not especially important for use as primary pharmaceutical packaging material, a value of approximately 5.0·10−6 K−1 should nevertheless be targeted, in order to set a resistance to thermal shocks comparable to that of the known neutral glasses such as SCHOTT FIOLAX® 8412. In addition, glasses having this thermal expansion can also be used as so-called sealing glasses in electrical engineering, since some metals and alloys likewise lie in this expansion range and therefore stable glass/metal composites, e.g. leadthroughs, are possible. When using Fe—Ni—Co alloys, such as VACON®, having a coefficient of thermal expansion a in the range of 20° C. to 300° C. of 5.4·10−6 K−1, zirconium (α20/300=5.9·10−6 K−1) or zirconium alloys, glasses having a coefficient of expansion α20/300 of between 5 and 6·10−6 K−1 are required as sealing glasses for glass/metal seals.
Although a range of boron-free glasses are known in the prior art, these are substantially unsuitable as neutral glasses in the sense of the present definition.
WO 96/39362 discloses a glass for boron-free glass fibers, comprising 59 to 62% by weight SiO2, 20 to 24% by weight CaO, 12 to 15% by weight Al2O3, 1 to 4% by weight MgO, 0 to 0.5% by weight F2, 0.1 to 2% by weight Na2O, 0 to 0.9% by weight TiO2, 0 to 0.5% by weight Fe2O3, 0 to 2% by weight K2O and 0 to 0.5% by weight SO3.
Although a glass of this type is suitable for the production of continuous glass fibers, it does not satisfy the demands imposed on a neutral glass.
U.S. Pat. No. 5,508,237 discloses a flat glass display comprising an aluminosilicate glass which exhibits a weight loss of less than 2.5 mg/cm2 after immersion for 24 hours in an aqueous 5% strength HCl solution at 95° C. The glass contains 49 to 67% by weight SiO2 and at least 6% by weight Al2O3, where Al2O3 is 6 to 14% by weight in conjunction with 55 to 67% by weight SiO2 and is 6 to 23% by weight in conjunction with 49 to 58% by weight SiO2. The total content of SiO2 and Al2O3 is greater than 68%. The glass further contains 0 to less than 8% by weight B2O3 and at least one alkaline earth metal oxide, specifically 0 to 21% by weight BaO, 0 to 15% by weight SrO, 0 to 7.1% by weight CaO, 0 to 8% by weight MgO, where the total content of BaO+CaO+SrO+MgO is 12 to 30% by weight.
The glass firstly does not have a sufficient acid resistance and secondly contains at least strontium oxide or barium oxide and possibly also boron oxide. It is therefore not suitable as boron-free neutral glass.
DE 10 2004 036 523 A1 discloses a glass substrate for a display which consists of a glass comprising 40 to 70% by weight SiO2, 2 to 25% by weight Al2O3, 0 to 20% by weight B2O3, 0 to 10% by weight MgO, 0 to 15% by weight CaO, 0 to 10% by weight SrO, 0 to 30% by weight BaO, 0 to 10% by weight ZnO, 0 to 25% by weight R2O (Li2O, Na2O, K2O), 0.4% by weight As2O3, 0 to 3% by weight Sb2O3 and 0.01 to 1% by weight SnO2. The glass is intended to be suitable for the production of flat glass using the downdraw method. In order to obtain a high acid resistance and a low coefficient of thermal expansion, the SiO2 content is preferably 57 to 64% by weight. In order to make it possible for the glass to be produced using the downdraw method or using the rotary method, the redraw method or the like, the glass has to have a sufficient flowability, and for this reason preferably 5 to 15% by weight B2O3, particularly preferably 7.5 to 11% by weight B2O3, are added. The glass preferably further contains strontium oxide and barium oxide.
A glass of this type is therefore not suitable as boron-free neutral glass which, in addition to a high acid resistance, also has to have a high hydrolytic resistance and alkali resistance.
U.S. Pat. No. 5,854,153 discloses a glass substrate for an electronic display, wherein the glass contains 42 to 62% by weight SiO2, 16.5 to 28% by weight Al2O3, 0 to 4% by weight B2O3, 3 to 10% by weight Na2O, 1 to 11% by weight K2O, 0 to 6% by weight MgO, 9.5 to 24% by weight CaO, 0.2 to 8% by weight SrO, 0 to 16% by weight BaO and 0 to 4% by weight ZrO2 and has a total alkali metal content of 4 to 16% by weight.
Owing to the low SiO2 content, a glass of this type does not have a sufficient chemical resistance.
Furthermore, EP 1 074 521 A2 discloses a boron-free glass composition for a filter medium, comprising 62 to 68 mol % SiO2, 2 to 6 mol % Al2O3, 10 to 16 mol % Na2O, 0 to 6 mol % K2O, 0 to 6 mol % Li2O, 3 to 10 mol % CaO, 0 to 8 mol % MgO, 0 to 3 mol % BaO, 2 to 6 mol % ZnO, 0 to 2 mol % TiO2 and 0 to 2 mol % F2, where the total alkali metal content is less than 18 mol %.
The glass is particularly suitable for the production of HEPA clean-room filters which consist of glass fibers. For this purpose, the glass has to have a relatively good acid resistance, although special emphasis is not placed on the hydrolytic resistance and the alkali resistance.
In practice, the known glass has an excessively low aluminium oxide content and an excessively high alkali metal content to be suitable as boron-free neutral glass.
WO 2008/143999 A1 discloses an alkali metal aluminosilicate glass comprising 64 to 68 mol % SiO2, 12 to 16 mol % Na2O, 8 to 12 mol % Al2O3, 0 to 3 mol % B2O3, 2 to 5 mol % K2O, 4 to 6 mol % MgO and 0 to 5 mol % CaO. In this glass, the total content of SiO2+B2O3+CaO is between 66 and 69 mol % and the total content of Na2O+K2O+B2O3+MgO+CaO+SrO is greater than 10 mol %. The total content of MgO+CaO+SrO is between 5 and 8 mol %. The difference resulting from the total content of Na2O+B2O3 less the Al2O3 content should be greater than 2 mol %, and the difference of Na2O−Al2O3 should be between 2 and 6 mol %. The difference resulting from the total content of Na2O+K2O less the Al2O3 content should be between 4 and 10 mol %.
In practice, the glass has an excessively high content of sodium oxide and potassium oxide to be suitable as neutral glass.